fix: avoid duplicate-key crash when listing events

[bryansray]

Summary

Fixes a crash in acal events list caused by duplicate calendarItemIdentifier values returned from EventKit.

Root cause

The EventKit-backed list path used Dictionary(uniqueKeysWithValues:) keyed only by calendarItemIdentifier. When EventKit returned multiple EKEvent values sharing that identifier, the command crashed with a fatal duplicate-key error.

What changed

• replaced trap-prone dictionary construction with explicit deduplication in the EventKit adapter
• deduplicate list rows by identifier plus occurrence timing instead of assuming one row per identifier
• populate occurrenceStart for EventKit list results
• mirror the same list dedupe behavior in the in-memory store

Repro

acal events list --from 2026-06-01 --to 2026-06-12

Previously this could fail with:

Swift/NativeDictionary.swift:792: Fatal error: Duplicate values for key: ...

Verification

• swift build
• swift build -c release
• swift test --parallel could not be completed in this environment because XCTest was unavailable to SwiftPM

Notes

I did not attach this work to a shared td issue because this public repo does not appear to include a shared td project database; .todos/ and .td-root are gitignored and local-only.

[gemini-code-assist]

Code Review

This pull request improves event deduplication in both InMemoryCalendarStore and EventKitAdapter by introducing a more comprehensive EventListIdentity struct that accounts for calendar IDs and occurrence start/end times. It also updates event serialization to include the occurrenceStart field. A compilation error was identified in EventKitAdapter.swift where .map is incorrectly called on a non-optional Date object; this should be replaced with a direct function call.

[tonydehnke]

Still reproducible in acal 0.4.0 through MCP on macOS 27.2 (26B5091g).

Observed in a read-only test:

• Calendar authorization reports full access.
• list_calendars succeeds.
• list_events for October 1–7, 2026, using explicit Vancouver -07:00 boundaries and limit: 3, terminates with Swift Fatal error: Duplicate values for key instead of returning events.

A couple of source-based details that may help callers until this lands:

• Reducing limit cannot prevent this crash. The MCP handler applies it only after store.listEvents returns.
• search_events also reaches the same list implementation, so changing to search does not bypass the duplicate-key construction.
• Calendar filtering happens before the EventKit fetch. A single-calendar, one-day query might avoid multiple occurrences with the same identifier, but that is an untested workaround, not a reliable fix; filtering by calendar alone will not separate occurrences within a recurring series.

This confirms the failure still affects the released 0.4.0 after the permission fix in #11. I have not built or runtime-tested this PR's patch.